The trend towards designing and building fuel efficient, low emission vehicles has increased dramatically over the last decade, this trend driven by concerns over the environment as well as increasing fuel costs. At the forefront of this trend has been the development of hybrid vehicles, vehicles that combine a relatively efficient combustion engine with an electric drive motor.
Currently, most common hybrids utilize a parallel drive system, although the implementation of the parallel drive system can vary markedly between different car manufacturers. In one form, illustrated in FIG. 1, power to wheels 101 is via planetary gears 103 and transaxle 105, the power coming from either, or both, combustion engine 107 and electric motor 109. A power splitter 111 splits the power from combustion engine 107 between generator 113 and the drive system, i.e., gears 103, axle 105 and wheels 101, the power split designed to maximize efficiency based on vehicle needs. The electric power generated by generator 113, after passing through an inverter 115, is used to either provide electricity to drive motor 109 or battery 117.
In hybrid system 100, motor 109 is the primary source of propulsion when the engine is relatively inefficient, for example during initial acceleration, when stationary, under deceleration or at low cruising speeds. Combustion engine 107 assists motor 109 in supplying propulsion power when demands on the vehicle are higher than what can be met by motor 109, for example during medium-to-hard acceleration, medium-to-high cruising speeds or when additional torque is required (e.g., hill climbing).
FIG. 2 illustrates the basic elements of another type of parallel drive system, often referred to as an integrated motor assist, or IMA, system. IMA system 200 utilizes a single electric motor 201 that is positioned between the combustion engine 203 and the drive system's transmission 205, transmission 205 coupling power through axle 207 to wheels 209. In this system motor 201 serves dual roles; first, as a drive motor and second, as a generator. In its capacity as a generator, motor 201 is coupled to battery pack 211 via inverter 213.
In hybrid system 200, engine 203 is the primary source of propulsion while motor 201 provides assistance during acceleration and cruising. During deceleration, motor 201 recaptures lost energy using a regenerative braking scheme, storing that energy in battery pack 211. As a result of this approach, a smaller and more fuel-efficient engine can be used without a significant lose in performance since motor 201 is able provide power assistance when needed.
Although in general hybrids provide improved fuel efficiency and lower emissions over those achievable by a non-hybrid vehicle, such cars typically have very complex and expensive drive systems due to the use of two different drive technologies. Additionally, as hybrids still rely on an internal combustion engine for a portion of their power, the inherent limitations of the engine prevent such vehicles from achieving the levels of pollution emission control and fuel efficiency desired by many. Accordingly several car manufacturers, including Tesla Motors, are studying and/or utilizing an all-electric drive system.
FIG. 3 illustrates the basic components associated with one configuration of an all-electric vehicle. As shown, EV 300 couples an electric motor 301 to axle 303 and wheels 305 via transmission/differential 307. A power control module 309 couples motor 301 to battery pack 311.
FIGS. 4 and 5 graphically illustrate some of the performance differences between a vehicle using a combustion engine as the sole propulsion source, one using hybrid technology, and one using only a single electric motor. In the torque curves shown in FIG. 4, curve 401 illustrates the narrow region over which a typical combustion engine provides torque, and thus the reason why multiple gears are required to utilize such an engine efficiently. Curve 501 in FIG. 5 is the corresponding power curve for the combustion engine. In a hybrid configuration, the output from a combustion engine is combined with an electric motor, thus combining the low speed torque provided by the electric assist motor (curve 403) with that of the combustion engine (curve 401) to provide a dramatic improvement in low speed torque. Curves 405 and 503 illustrate the torque and power, respectively, of such a combination. Curves 407 and 505 illustrate the benefits of a high output power, all electric drive system, specifically showing both the low speed torque/power that such a system provides as well as the wide speed range over which such torque/power is available.
Although significant advancements have been made in the area of fuel efficient, low emission vehicles, further improvements are needed. For example, hybrid vehicles still rely on combustion engines for a portion of their power, thus not providing the desired levels of fuel independence and emission control. Current all-electric vehicles, although avoiding the pitfalls associated with combustion engines, may not have the range, power or level of traction control desired by many. Accordingly, what is needed is an improved all-electric vehicle drive system. The present invention provides such a system.